A set (pitch set, pitch-class set, set class, set form, pitch collection) in music theory, as in mathematics and general parlance, is a collection of objects. In musical contexts the term is traditionally applied most often to collections of pitches or pitch-classes, but theorists have extended its use to other types of musical entities, so that one may speak of sets of durations or timbres, for example.
A set by itself does not necessarily possess any additional structure, such as an ordering. Nevertheless, it is often musically important to consider sets that are equipped with an order relation (called segments); in such contexts, bare sets are often referred to as "unordered", for the sake of emphasis.
Two-element sets are called dyads, three-element sets trichords (occasionally "triads", though this is easily confused with the traditional meaning of the word triad). Sets of higher cardinalities are called tetrachords (or tetrads), pentachords (or pentads), hexachords (or hexads), heptachords (heptads or, sometimes, mixing Latin and Greek roots, "septachords"—e.g.,), octachords (octads), nonachords (nonads), decachords (decads), undecachords, and, finally, the dodecachord.
In the theory of serial music, however, some authors[weasel words] (notably Milton Babbitt) use the term "set" where others would use "row" or "series", namely to denote an ordered collection (such as a twelve-tone row) used to structure a work. These authors[weasel words] speak of "twelve tone sets", "time-point sets", "derived sets", etc. (See below.) This is a different usage of the term "set" from that described above (and referred to in the term "set theory").
For these authors,[weasel words] a set form (or row form) is a particular arrangement of such an ordered set: the prime form (original order), inverse (upside down), retrograde (backwards), and retrograde inverse (backwards and upside down).
B B♭ D E♭ G F♯ G♯ E F C C♯ A
Represented numerically as the integers 0 to 11:
0 11 3 4 8 7 9 5 6 1 2 10
The first subset (B B♭ D) being:
0 11 3 prime-form, interval-string = <-1 +4>
The second subset (E♭ G F♯) being the retrograde-inverse of the first, transposed up one semitone:
3 11 0 retrograde, interval-string = <-4 +1> mod 12 3 7 6 inverse, interval-string = <+4 -1> mod 12 + 1 1 1 ------ = 4 8 7
The third subset (G♯ E F) being the retrograde of the first, transposed up (or down) six semitones:
3 11 0 retrograde + 6 6 6 ------ 9 5 6
And the fourth subset (C C♯ A) being the inverse of the first, transposed up one semitone:
0 11 3 prime form, interval-vector = <-1 +4> mod 12 0 1 9 inverse, interval-string = <+1 -4> mod 12 + 1 1 1 ------- 1 2 10
Each of the four trichords (3-note sets) thus displays a relationship which can be made obvious by any of the four serial row operations, and thus creates certain invariances. These invariances in serial music are analogous to the use of common-tones and common-chords in tonal music.
The normal form of a set is the most compact ordering of the pitches in a set. Tomlin defines the "most compact" ordering as the one where, "the largest of the intervals between any two consecutive pitches is between the first and last pitch listed". For example, the set (0,2) (a major second) is in normal form while the set (0,10) (a minor seventh, the inversion of a major second) is not, its normal form being (10,0).
Rather than the "original" (untransposed, uninverted) form of the set the prime form may be considered the either the normal form of the set or the normal form of its inversion, whichever is more tightly packed.
See also 
Further reading 
- Schuijer, Michiel (2008). Analyzing Atonal Music: Pitch-Class Set Theory and Its Contexts. ISBN 978-1-58046-270-9.
- Whittall, Arnold (2008). The Cambridge Introduction to Serialism, p.165. New York: Cambridge University Press. ISBN 978-0-521-68200-8 (pbk).
- Wittlich, Gary (1975). "Sets and Ordering Procedures in Twentieth-Century Music", Aspects of Twentieth-Century Music, p.475. Wittlich, Gary (ed.). Englewood Cliffs, New Jersey: Prentice-Hall. ISBN 0-13-049346-5.
- Whittall (2008), p.127.
- Morris, Robert (1987). Composition With Pitch-Classes: A Theory of Compositional Design, p.27. Yale University Press. ISBN 0-300-03684-1.
- Rahn (1980), 140.
- Wittlich (1975), p.476.
- See any of his writings on the twelve-tone system, virtually all of which are reprinted in The Collected Essays of Milton Babbitt, S. Peles et. al, eds. Princeton University Press, 2003. ISBN 0-691-08966-3.
- Wittlich (1975), p.474.
- Whittall (2008), p.97.
- John Rahn, Basic Atonal Theory (New York: Longman; London and Toronto: Prentice Hall International, 1980), pp.27–28. ISBN 0-582-28117-2 (Longman); ISBN 0-02-873160-3 (Prentice Hall International). Reprinted 1987 (New York: Schirmer Books; London: Collier Macmillan, 1980), p.27. ISBN 0-02-873160-3.
- Tomlin, Jay. "All About Set Theory: What is Normal Form?", JayTomlin.com.
- Tomlin, Jay. "All About Set Theory: What is Prime Form?", JayTomlin.com.
- "Set Theory Calculator", JayTomlin.com. Calculates normal form, prime form, Forte number, and interval class vector for a given set and vice versa.